This invention relates to a weighing device which is set on a floor and comprises a load cell for detecting the floor vibrations in the vertical direction.
A weighing device is subjected to many kinds of vibrations, wherever it is installed, due to environmental vibrations of the ground, building, floor and/or the table (hereinafter generally referred to as the floor vibrations). When objects are weighed by such a weighing device, components of such vibrations are added to the weight signals outputted by the weighing device. In order to effect high-speed, high-precision weighing, it is essential to make corrections on weight signals by subtracting such vibration components therefrom.
Since floor vibrations generally have lower frequencies than the mechanical vibrations which are caused when an object to be weighed is placed on the weighing device, a low-pass filter with a low cutoff frequency may be used in order to eliminate the components of floor vibrations from weight signals, but the response becomes slower if such a low-pass filter is used, and the overall efficiency of the weighing process is adversely affected. For this reason, there have been attempts at eliminating the floor vibration components in some other way such that the cutoff frequency of the filter can be set higher and the speed of weighing can be increased.
FIG. 11 is a block diagram of a prior art weighing device having a weight-measuring load cell (or a scale cell) 56 and a vibration-detecting load cell (or a detection cell) 51 installed on the same floor F. Analog weight signals indicative of weights measured by and outputted from the scale cell 56 are passed through an amplifier 57 and converted into digital signals by an analog-to-digital converter (A/D) 58. Effects of mechanical vibrations are removed therefrom by a digital filter 59 before the digital weight signals are inputted to a subtractor 60. Analog signals indicative of the vibrations of the floor F outputted from the detection cell 51 are similarly processed, that is, they are passed through an amplifier 52, converted into digital signals by an analog-to-digital converter (A/D) 53 and inputted to the subtractor 60 after appropriate adjustments are effected thereon by an adjusting means 55. As explained, for example, in U.S. Pat. No. 5,117,929 issued Jun. 2, 1992, the adjusting means 55 may comprise a multiplier for making adjustments necessitated by the difference in sensitivity between the scale cell 56 and the detection cell 51. The subtractor 60 serves to subtract the effects of the floor vibrations detected by the detection cell 51 from the weight signals from the scale cell 56 to thereby output corrected weight signals which are free from the effects of the floor vibrations.
It now goes without saying that the A/D converters 53 and 58 must be able to convert vibration components of the floor F accurately and without allowing them to go over their range because, if otherwise, correct values would not be obtained after the signals are passed through the digital filters 54 and 59. Now, the usable dynamic range of the A/D converters 53 and 58 (or the range of input voltage determined by the number of bits corresponding to the resolution) is determined by their frequency (f) characteristic as shown by the log-log graph in FIG. 12, wherein V.sub.ref indicates the output when the transfer function is 1. In general, if the amplitude of the floor vibration, the relative displacement of an object set on the floor with respect thereto and the attenuation constant of the vibration are denoted by A.sub.0, Y and .zeta., respectively, the relationship (Y/A.sub.0)=1/(2.zeta.) holds at the characteristic frequency f.sub.0 (such as 90 Hz). Accordingly, the peak value P at the characteristic frequency f.sub.0 becomes higher if the attenuation constant .zeta. is made smaller, and the upper limit of the usable dynamic range of the A/D converters 53 and 58 is determined by this peak value P.
If a large force including high frequency components is experienced from the floor, a very large signal proportional to the peak value P is outputted from the detection cell 51. In order to convert such a signal into a digital signal without allowing it to go beyond the range of the A/D converter 53, its range of input voltage must be increased accordingly. Since there is a limit to the resolution (or the number of bits) of the A/D converter 53, however, this means that the resolution of the converter 53 becomes low in the lower frequency range of interest, and hence that the effects of floor vibrations cannot be eliminated accurately. One method of obtaining sufficiently high resolution while keeping the range of input voltage wide enough would be to reduce the peak value P. If the peak value P is reduced (say, to P.sub.1 as shown in FIG. 12), the resolution becomes higher and accurate detection becomes possible in a region of lower floor vibration frequencies (say, of about 5-15 Hz shown at 101).
As a means for reducing the peak value P, it has been known to connect an oil dumper (as disclosed, for example, in Japanese Utility Model Publication Jikko 59-593, and shown at 61 in FIG. 11) to the detection cell 51 to thereby increase its vibration attenuation constant and to increase its usable dynamic range.
Weighing devices using such a prior art detecting cell are difficult to use, however, because it is cumbersome to attach a dumper to the detecting cell and it is difficult to set the oil damper appropriately such that a desired detection attenuation constant is obtained. Moreover, the use of an oil dumper makes it difficult to design a compact detecting cell because of limitations placed on the shape of the oil dumper.